Wednesday, September 30, 2015

"Hurricane Warnings for the Bahamas From Joaquin; Threat to U.S. East Coast Grows"

Joaquin
is now a hurricane, and Hurricane Warnings are up for the Central
Bahama Islands as the slowly intensifying storm moves southwest at 6
mph. An Air Force hurricane hunter aircraft made two penetrations of
Joaquin's center on Wednesday morning, and found top surface winds of 80
mph, a central pressure of 971 mb, and a huge 54-mile diameter eye with
a fully closed eyewall. Joaquin continues to battle high wind shear
of 20 knots due to strong upper-level winds out of the north-northwest,
but this wind shear had fallen by about 5 knots since Tuesday morning. Water vapor satellite loops
show that a large area of dry air lay to the northwest of Joaquin, and
the strong wind shear was driving this dry air into Joaquin's core,
keeping intensification slow. Visible and infrared satellite loops
show that Joaquin has developed a large Central Dense Overcast (CDO) of
high cirrus clouds over the center, characteristic of intensifying
storms, and the hurricane's large eye was beginning to be apparent. Upper level winds analyses from the University of Wisconsin
show that the hurricane has developed an impressive upper-level outflow
channel to the southeast, which is supporting the intensification
process. Ocean temperatures in the region are near 30°C (86°F)--the warmest seen there since record keeping began in 1880.

The U.S. outlook for Joaquin
A hurricane watch
could be required for portions of the U.S. East Coast as early as
Thursday night. The forecast for Joaquin is very complex, and the
confidence in both the intensity and track forecast for the storm is
low. Joaquin is trapped to the south of a high pressure system whose
clockwise flow will push the cyclone very slowly to the southwest or
west-southwest at about 3 - 6 mph. As the storm progresses to the
southwest, the strong upper-level winds out of the north currently
bringing high wind shear of 20 knots will gradually decrease, continuing
to allow Joaquin to strengthen. The 8 am EDT Wednesday run of the SHIPS model predicted that wind shear over Joaquin would fall to the moderate
range, 10 - 20 knots, on Thursday and Friday. These conditions should
allow Joaquin to intensify to a Category 2 hurricane by Thursday. As
Joaquin progresses to the west, the storm will also increasingly "feel"
the steering influence of a strong upper-level trough of low pressure
situated over the Eastern United States on Friday, and begin to turn
north. These winds may also open up another upper-level outflow channel
to the northwest of Joaquin on Friday, potentially allowing the storm to
intensify to Category 3 strength. However, as Joaquin gets closer to
this trough, its winds will bring high wind shear of 20+ knots, likely
halting the intensification process and causing weakening by Sunday....MORE

Figure 2.
Our two top models for forecasting hurricane tracks, both run at 8 pm
EDT Tuesday September 29, 2015 (00Z Wednesday) , came up with two very
different solutions for the path of Joaquin. The GFS model showed
Joaquin making landfall in Virginia, while the European model took the
storm to the northeast out to sea without hitting the U.S. Image credit:
wundermap with the "Model Data" layer turned on.